This proposal concerns the development and analysis of mouse models for the study of Trk protooncogene receptors and their cognate ligands, the NGF-related neurotrophins. Neurotrophic factors perform critical functions in the development and survival of diverse neurons and glia during vertebrate embryonic development. Recent evidence indicates that neurotrophins continue to support nerve cell survival and regeneration in adults CNS and in peripheral motor neurons. These results imply that neurotrophins may have pharmacological potential in nerve regeneration. The correlation between neuronal NGF dependence, NGF (trk ) receptor expression and neuronal death observed in Alzheimer's patients, suggests that neurotrophins may also have potential therapeutic value in neurodegenerative diseases. The availability of reliable in vivo models to study neurotrophin function is of critical importance for further evaluation of neurotrophin sites and mechanisms of activity. The Trk receptor tyrosine kinase gene family encodes the functional receptors for the neurotrophins. This observation has opened new areas of signal transduction and biochemistry to the field. However several complexities remain unanswered and the potential for neurotrophin therapy via intervention of their cognate receptor awaits further understanding of the system. Thus, while the different receptor/ligand interactions for the different Trk//neurotrophin pairings can be assessed in cell culture systems, the ultimate significance of these interactions is best addressed in the organism. We intend to generate mouse lines harboring targeted null mutations in the following genes: trk, trkB, trkC, BDNF, NT-3 and NT4/5 through ES cell technology. The consequences of these mutations, in the homozygous state, for neuroblast development and survival will be analyzed during development of the nervous system. Direct comparison of the different mutations, and genetic intercrossing of the various mutations, will provide critical information concerning the importance and developmental timing of the different receptor/ligand interactions that have been observed in vitro. The spontaneous neurological mutations spastic and pcd map to the same chromosomal regions as do trk and trkB respectively. Genetic crosses between the receptor mutants and the spontaneous mutants will allow us to determine whether spa and pcd are alleles of the trk and trkB genes. Trk family genes encode various isoforms including receptors lacking the kinase domains and receptors harboring kinase domain insertions. The functions of these isoforms are currently unknown. We propose to generate targeted mutations in the trkB and trkC genes that specifically ablate these isoforms as a means of creating a system that will permit study of their functions. Recognizing the potential value of the mouse strains developed in our study, to the fields of cancer biology, aging, nerve disease, the and spinal cord injury , among others, we will propagate our stocks and distribute them to investigators in the basic research community.